Recently, a demand for a larger screen has been on the rise in a display device adopted in audio-visual (AV) equipment and office automation (OA) equipment, etc. In order to meet such a demand, the cathode ray tube (CRT) system, a liquid crystal display device (LCD), a plasma display device (PDP), an electroluminescence (EL) display device, and a light-emitting diode (LED) display device are under extensive research to realize an actual application of those devices.
Of those devices, the liquid crystal display device in particular has such an advantage in that (1) the thickness (depth) can be made significantly thinner compared with other display devices, (2) power consumption is low, and (3) a full-color image can be obtained with ease. For these reasons, the liquid crystal display device has been applied in a wide variety of fields, and is considered to be the best candidate for realizing a larger screen.
However, in the liquid crystal display device, when the screen is to be made larger, in the manufacturing process, the fraction defective induced by breakage of signal lines and a pixel failure is conspicuously increased. This presents a problem of a rise in cost.
The following will explain such a problem in detail using as an example an active-matrix-type liquid crystal display device which is most widely adopted. The active-matrix-type liquid crystal display device has an arrangement wherein liquid crystal is enclosed between a pair of substrates. One of the pair of substrates constitutes an active-matrix substrate on which a small active element such as a TFT (Thin Film Transistor) or an MIM (Metal-Insulator-Metal) is formed for each pixel. Thus, when the active matrix substrate is made larger, the active element and surrounding circuits become susceptible to breakage and failure. As a result, the fraction non-defective is conspicuously reduced.
As a method for solving this problem, a method for realizing a larger screen liquid crystal display device has been suggested in which a substrate is prepared by connecting to each other a plurality of small substrates on the sides, and the substrate thus prepared is faced with a large substrate sandwitching a liquid crystal layer therebetween.
For example, Japanese Publication for Unexamined Utility Model No. 191029/1985 (Jitsukaishou 60-191029) discloses a method for realizing a large screen liquid crystal display device in which, as shown in FIG. 7, (1) a large substrate 53 composed of small active matrix substrates (referred to as small substrates hereinafter) 53a provided in a 2.times.2 arrangement on which pixel electrodes 51 are formed in matrix, and (2) a large substrate (referred to as counter substrate hereinafter) 54, on which counter electrodes (not shown) are formed, having a size substantially the same as the large substrate 53 are faced each other so as to be combined with a seal material 55.
In this arrangement, as shown in FIG. 8, the small substrates 53a are connected to each other on the sides by a transparent bonding agent 56 so that the small substrates 53a thus connected to each other constitute the large substrate 53, and between the large substrate 53 and the counter substrate 54, a liquid crystal layer 57 is formed as liquid crystal is enclosed by the seal material 55 therebetween. Also, spacers 58, which determine the thickness (cell gap) of the liquid crystal layer 57, are spread over the region surrounded by the seal material 55.
In the liquid crystal display device having the described arrangement, lowering of the fraction non-defective in response to a larger screen can be prevented, and therefore the productivity in the manufacturing process can be increased.
However, according to the method disclosed in Japanese Publication for Unexamined Utility Model No. 191029/1985 (Jitsukaihei 60-191029), as described above, in order to combine the counter substrate 54 and the large substrate 53 with each other, it is required beforehand to connect the small substrates 53a to each other on the sides so as to obtain the large substrate 53. Generally, as the small substrates 53a, a glass substrate having a thickness of 0.7 mm or 1.1 mm is adopted. Thus, even when the small substrates 53a are connected to each other on the sides by the bonding agent 56, because the bonding area is so small that the strength of the connecting portion connecting the small substrates 53a is extremely weak.
For example, in the case where two or four small substrates 53a each having an area of 300 mm.times.400 mm are connected, the small substrates 53a are connected to each other on the sides having a thickness of mere 0.7 mm or 1.1 mm. Thus, the strength of the connecting portion is extremely weak so that the connecting portion is destroyed by only a small amount of an external force. This presents a problem that the liquid crystal display device must be handled with great care not only in the manufacturing process but also as a product after the manufacturing process.
Further, on the large substrate 53 prepared by connecting to each other the small substrates 53a by the bonding agent 56, only a 2 .mu.m to 3 .mu.m step-difference at the connecting portion on the side contacting with the liquid crystal layer 57 changes the cell gap at the connecting portion. The change in cell gap caused in this manner adversely affects the display image.
For example, when the thickness of the liquid crystal layer 57 is 5 .mu.m, a step-difference of only 2 .mu.m to 3 .mu.m at the connecting portion makes the thickness of the liquid crystal layer 57 substantially in half. For this reason, a change in cell gap is significant at the connecting portion compared with other regions. This presents a problem of nonuniformity in displayed colors, and the display image being adversely affected.
In order to solve above-mentioned problems, Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849) discloses a method for realizing a large screen liquid crystal display device wherein, as shown in FIG. 9(a) and FIG. 9(b), in a liquid crystal display device in which four small substrates 61a are connected to each other in a 2.times.2 arrangement so as to prepare a large substrate 61, and in which the large substrate 61 thus prepared and a counter substrate 62 are combined with each other so as to enclose therebetween a liquid crystal layer 64 by a seal material 63, a support base 66 is provided between the connecting portion of the small substrates 61a and a non-translucent patterning 65 formed on the counter substrate.
Specifically, in this method, as shown in FIG. 10(a), the seal material 63 and the non-translucent patterning 65 are provided on a counter substrate 62. Note that, the non-translucent patterning 65 is formed on a portion corresponding to the connecting portion of the small substrates 61a. On the non-translucent patterning 65, the support base 66 made of the same material as that of the seal material 63 is provided. Also, on a region surrounded by the seal material 63, gap materials 68 made of, for example, plastic beads are spread over. The gap materials 68 are also included in the seal material 63 and the support base 66.
Then, as shown in FIG. 10(b), the small substrates 61a are combined one by one with the counter substrate 62, and as shown in FIG. 10(c), the bonding agent 67 is injected into the connecting portion of the small substrates 61a so as to connect the small substrates 61a to each other, thereby obtaining a liquid crystal display device.
Thus, in the liquid crystal display device manufactured in this manner, the support base 66 is provided between the connecting portion at which the small substrates 61a are connected to the bonding agent 67 and the non-translucent patterning 65 facing the connecting portion. With this arrangement, it is possible to prevent the fraction non-defective from lowering in response to making the screen of the liquid crystal display device large, and therefore the productivity can be increased, and bending generated with time on end portions of the small substrates 61a can be prevented. It is also possible to level the cell gap at the connecting portion of the small substrates 61a.
However, in the method disclosed in Japanese Unexamined Patent publication No. 184849/1996 (Tokukaihei 8-184849), the following problems may be presented in the manufacturing steps of a liquid crystal panel.
In the method disclosed in the above publication, as shown in FIG. 11, when combining the small substrates 61a and the counter substrate 62, there is a chance that the support base 66 intrudes into a gap 69 at the connecting portion of the small substrates 61a. As the support base 66 and the seal material 63, colored resin such as white or light brown is widely adopted. For this reason, when such resin is present at the connecting portion between the small substrates 61a made of transparent glass, the light passing through the connecting portion is colored by being refracted or scattered. This adversely affects the display image.